1. Field of the Invention
The present invention relates generally to an electric power steering apparatus, and more particularly to an improvement in a rack-and-pinion mechanism used in such an electric power steering apparatus.
2. Description of the Related Art
Electric power steering systems are commonly used to make steering easier by reducing a force needed to turn a steering wheel (referred to as the steering force below). Electric power steering systems use an electric motor to produce assist torque according to the steering torque, and transfer the assist torque to the rack-and-pinion mechanism of the steering system, as taught in, for example, Japanese Patent Laid-Open Publication (kokai) No. SHO-61-160359.
The disclosed electric power steering apparatus includes a single rack shaft designed for meshing engagement with first and second pinions to thereby steer right and left steered wheels. Steering torque produced by turning a steering wheel is transmitted via the first pinion to the rack shaft while an assist torque produced by an electric motor is transmitted via the second pinion to the rack shaft. The rack shaft steers the steered wheels by the combined steering torque and assist torque. In the conventional electric power steering apparatus, since the rack-and-pinion mechanism for transmitting the steering torque is separated from the rack-and-pinion mechanism for transmitting the assist torque, each rack-and-pinion mechanism can advantageously be made to have smaller strength than a unified rack-and-pinion mechanism.
An automotive steering system also usually has a stopper mechanism for limiting the maximum turning angle of the steering wheels. More specifically, the stopper mechanism has a rack end stopper attached at each longitudinal end of the housing in which the rack shaft is slidably disposed, and a ball joint, for example, is attached to each end of the rack shaft. When the rack shaft slides a specific distance, the ball joint contacts the rack end stopper. The maximum turning angle of the steering wheels is thus limited by limiting the movement of the rack shaft.
As the rack shaft is slid a specific distance, its further movement is restricted by the stopper mechanism. Upon stoppage of the rack shaft, the second pinion is fed with a torque proportionate to the square of a reduction gear ratio due to motor inertia and is thus supplied with a larger assist torque than it is in a normal operation. The assist torque becomes maximum at this time and larger than the steering torque. Consequently, the second rack-and-pinion mechanism needs to have strength sufficient to withstand the maximum torque. For this purpose, one may propose to make each component have increased strength but this requires a larger rack-and-pinion module and high quality materials, thereby rendering the rack-an-pinion mechanism large in size and expensive.
It is therefore an object of the present invention to provide an electric power steering apparatus including a first rack-and-pinion mechanism for transmitting a steering torque and a separate second rack-and-pinion mechanism for transmitting an assist torque with strength and durability sufficient to with stand a torque load of motor inertia.
According to an aspect of the present invention there is provided an electric power steering apparatus which comprises: a rack shaft for steering wheels, the rack shaft having a first rack and a second rack provided separately axially thereof; a first rack-and-pinion mechanism for transferring a steering torque, produced by turning a steering wheel, to the rack shaft; an electric motor for producing an assist torque in accordance with the steering torque; and a second rack-and-pinion mechanism, comprised of a pinion and the second rack, for transferring via a geared reduction mechanism the assist torque to the rack shaft, the pinion and rack of the second rack-and-pinion mechanism both being helical gears, one of the helical gears having a tooth profile wherein at least an a dedendum is a circular arc generally centered on a reference pitch line, the other of the helical gears having a tooth profile wherein at least a addendum is a circular arc generally centered on the reference pitch line.
With the rack and pinion being formed of helical gears, the second rack-and-pinion mechanism can transfer a larger torque than a conventional spur gear.
The tooth profile of the pinion and rack of the second rack-and-pinion mechanism of the present invention is a curved arc. Because a conventional involute tooth profile is convex, meshing in a gear pair is contact between two convex surfaces. With the curved arc tooth profile of the present invention, however meshing in a gear pair occurs as contact between a convex surface and a concave surface. The contact area is thus increased, whereby contact pressure is reduced to approximately ⅙ that of an involute tooth profile. By thus using a curved arc tooth profile in the rack and pinion of the second rack-and-pinion mechanism, surface fatigue strength, bending strength, and bending fatigue strength are greater than with an involute tooth profile. This means that the rack-and-pinion mechanism of our invention can transfer the assist torque sufficiently, even when the assist torque from the motor is larger than that in a normal operation of the motor.
Since it transfers only a driver""s steering torque, the first rack-and-pinion mechanism is not fed with a steering torque extremely large compared with one in normal driving conditions, even when the rack shaft is stopped. It is thus not necessary to increase rigidity of the mechanism.
When the steered wheels turn right or left to the maximum steering angle and the rack shaft meets the rack end stopper, that is, when the rack shaft moves to the end of its range of movement, the rack drops immediately. Because the torque at this time is impact torque and not static torque, it is significantly higher than during normal driving conditions. However, because the helix angle of the helical gear pinion is less than the helical gear friction angle, thrust does not act on the pinion. Thrust acting on the pinion is only an extremely weak force occurring during normal conditions when the rack is not stopped at the right or left end of its range.
The geared reduction mechanism of the present invention is preferably a combination of driver and driven gears in which the tooth surfaces of the driver gear and/or the tooth surfaces of the driven gear are coated with a low friction material coating, and the driver gear and driven gear mesh with no backlash. Coating with a low friction coefficient material can be achieved by imparting a coating made from a low friction coefficient material, or by impregnating the tooth surfaces with a low friction coefficient material.
By thus meshing driver gear and driven gear with no backlash, there is no play between the driver and driven gears, and impact torque due to motor inertia does not pass from the driver gear tooth surface to the driven gear tooth surface. Moreover, the tooth surfaces of one or both of the driver gear and driven gear are coated with a low friction coefficient material coating. By lowering the coefficient of friction between the tooth surfaces of the driver and driven gears by means of this coating, power transfer efficiency can be increased even though there is no play between the driver and driven gears.
It is further preferable to insert a torque limiter between the motor and the geared reduction mechanism to limit the transfer of assist torque exceeding a specific limit from the motor to the reduction mechanism. When the rack shaft hits the rack end stopper, excessive torque will not be produced as a reaction to the motor, and excessive torque will not be transferred to the load side.
It is yet further preferable to provide a steering torque sensor for detecting steering torque. Yet further preferably the steering torque sensor is a magnetostrictive sensor for detecting magnetostriction of the pinion shaft of the rack-and-pinion mechanism. By using such a steering torque sensor, it is not necessary to divide the input shaft into two parts lengthwise and connect these two parts using a torsion bar as It is when steering torque is detected using the method of a conventional electric power steering apparatus. It is therefore also possible to lengthen the input shaft. Machining precision is increased by lengthening the pinion shaft, and the pinion and rack thus mesh more precisely. There is a particularly strong correlation between meshing precision and power transfer efficiency in a rack-and-pinion mechanism having a curved arc tooth profile, and improving meshing precision is therefore important.
The pinion and/or rack of the rack-and-pinion mechanism in the present invention is yet further preferably a forging or other plastically processed part. There are, therefore no process marks left on the tooth surface as there are when the tooth surfaces are conventionally machined, and the surface roughness of the gear teeth is smooth. Friction force from sliding gear tooth surfaces is thus reduced, and the power transfer efficiency of the rack-and-pinion mechanism is increased.
Furthermore, because the pinion and rack are plastically processed parts, there is no residual stress produced in the tooth surfaces as there is with machining processes, and there is thus less deformation during hardening. A good tooth surface with low strain can therefore be achieved without correcting the tooth profile after hardening. In other words, because these parts are plastically processed, the surface roughness condition of the teeth is good with little strain from hardening or tool marks left. In addition, strength is increased because a fiber structure flowing continuously along the tooth profile is achieved through plastic processing, and bending strength and wear resistance are greater compared with machined gears in which the fiber structure is interrupted.
By processing the teeth of the rack and pinion to a curved arc tooth profile, and achieving this curved arc tooth profile in the rack and pinion by means of forging or other plastic processing technique, contact pressure is reduced, a good surface roughness condition is achieved, and interruption of the oil membrane formed by the lubricating fluid can be prevented. An electric power steering apparatus with little motor output loss can thus be provided because contact resistance between tooth surfaces can be significantly reduced and the power transfer efficiency of the rack-and-pinion mechanism improved.
Furthermore, by using forgings or otherwise plastically processed components for the curved arc tooth profile pinion and rack, it is possible to provide an electric power steering apparatus featuring improved mechanical properties in the materials, less tooth base stress, reduced wear, and outstanding strength and durability.
Yet further preferably, the rack shaft to which the rack is formed is comprised so that the back on the side opposite that to which the rack is formed is pushed toward the pinion by an adjustment bolt by way of intervening rack guide member and compression spring, particularly so that the adjustment bolt pushes directly against the back of the rack guide member when the pinion and rack mesh.
Good meshing between the pinion and rack can be maintained as a result of the rack guide member constantly pushing the rack shaft to the pinion, and the power transfer efficiency of the rack-and-pinion mechanism can thus be stabilized. Assist torque from the motor can be particularly transferred efficiently from the pinion to the rack shaft even during high load conditions such as turning the wheels when the vehicle is stopped. Therefore, compared with using a conventional involute tooth profile, less assist torque is needed, and a low power consumption electric power steering apparatus can be provided.
Moreover, tooth surface wear is reduced because the curved arc tooth profile is formed by forging or other plastic processing method. It is therefore possible to provide an electric power steering apparatus having a rack-and-pinion mechanism with little play even without applying pressure using an adjustment spring
Furthermore, because the tooth profile of the rack and pinion is a curved arc as described above, the contact area of meshed teeth is greater than that with an involute tooth profile. Because the contact pressure drops, tooth surface sliding is also smoother. A good steering feel can also be maintained in the steering wheel even though an adjustment bolt directly supports the rack shaft so that the rack shaft will not move back in reaction to the strong force produced perpendicular to the longitudinal axis when high torque due to motor inertia acts on the rack-and-pinion mechanism.
The rack shaft on which the rack is formed is housed unrockably and slidably in the longitudinal direction in a housing. A rocking force is produced on the rack shaft when the pinion and rack are helical gears, but this rocking action of the rack shaft is restricted in the present invention. Good meshing between the pinion and rack can thus be maintained.
More specifically, the back of the rack shaft opposite the surface on which the rack is formed is convex, and a rack guide is disposed having a concave end for contacting convex back at contact points, and pushing the convex back of the rack shaft toward the rack. These contact points are set in relation to the rack shaft supported by the housing so the concave end limits rocking of the convex part of the rack shaft when a rocking force acts on the rack shaft. The rack shaft is thereby housed so that it cannot rock in the housing.
The rack guide preferably pushes the guide member having the concave end to the rack shaft side by means of adjustment bolt and intervening compression spring. The adjustment bolt pushes directly on the back of the surface to which the concave end is formed to the guide member when the pinion and rack mesh.
When torque is transferred from the pinion to the rack during steering, forces act on the rack shaft in the direction of the longitudinal axis and in the direction perpendicular thereto. Because the adjustment bolt pushes directly against the back of the guide member, the rack cannot move back as a result of force in the longitudinal axis direction. Good meshing between the pinion and rack can thus be always maintained. Moreover, the contact area is great and contact pressure between meshing surfaces is reduced as a result of the curved arc tooth profile, and sliding between the tooth surfaces is therefore smoother.
Yet further preferably, a supported part whereby the rack shaft is supported on a housing by way of intervening bearings, and a rack formation part to which the rack is formed, are disposed to the rack shaft. The section perpendicular to the axis of the rack formation part is a circular section equal in diameter to the supported part, and the distance from the center of this circular section to the reference pitch line is set to a specific dimension. The actual tooth width of the rack is greater than the rack tooth width determined by this specific dimension.
By thus making the tooth width of the rack actually greater than the tooth width of a conventional rack, the mechanical strength (bending strength and bearing strength) of the rack is improved, and a rack-and-pinion mechanism with strength sufficient to withstand the torque load from motor inertia can be achieved. The part of the rack shaft where the rack is not formed only needs rigidity comparable to a conventional rack shaft because it simply slides to push the wheels for steering. The weight of the rack shaft can also be limited because it is only necessary to increase the tooth width of the rack.
It is further preferable to make the tooth width of the rack formed on the rack shaft greater than the diameter of the rack shaft in that part where the rack is not formed.